In a typical gas turbine engine supplying mechanical output through a rotating shaft connected to a driven device, a compressor wheel ingests and compresses an oxidant, such as ambient air, which is then mixed with atomized fuel and burned in a combustor to produce hot gases of combustion. The hot gases of combustion are then directed at and contained against a turbine wheel by an annular nozzle and an annular shroud, respectively, to cause rotation of the turbine wheel which is operatively connected to drive the rotating shaft.
It is well known that, in order to maximize fuel efficiency and power output from such a gas turbine engine, the engine should be operated with a combustion temperature and a turbine inlet temperature which are as high as possible. As a practical matter, however, the maximum temperatures which may be utilized are determined by the ability of materials used in fabricating components of the engine, such as the combustor, turbine wheel, nozzle, and shroud, to withstand extended exposure to elevated temperatures.
While it is not possible to completely overcome the limitations on combustion and turbine inlet temperatures which are imposed by the materials, it is well known in the art that an acceptable balance between power output, reliability, and life of the engine may be achieved by utilizing a relatively high combustion temperature and providing means within the engine for utilizing a portion of the compressed oxidant either as a diluent injected just upstream of the nozzle for reducing the temperature of the hot gases, or for convectively cooling engine components exposed to the hot gases.
Examples of such means for utilizing the compressed oxidant as a diluent or for convective cooling are disclosed in U.S. Pat. Nos. 4,825,640, 4,926,630, 4,944,152, 4,949,545, and 5,033,263. In each of these patents, the compressed oxidant exits the compressor wheel through an annular outlet which is connected in fluid communication with an annular inlet to an annular combustor. The combustor includes a liner which defines an annular combustion chamber having an annular outlet in fluid communication with the nozzle. The combustor further includes a housing which is spaced from and surrounds the liner such that a plenum chamber having an annular inlet in fluid communication with the inlet to the combustor and an outlet in fluid communication with the combustion chamber is formed between the liner and the housing. Compressed oxidant flowing within the plenum is utilized to provide convective cooling of the shroud and the liner, thereby allowing the use of a high combustion temperature. A portion of the compressed oxidant is also injected into the combustion chamber to dilute, and thus lower the temperature of, the hot gas just upstream from the nozzle such that, although a high combustion temperature is utilized to enhance combustion, the turbine wheel and the shroud are exposed to a somewhat lower inlet temperature which is conducive to achieving improved reliability and longer engine life.
In gas turbine engines such as those cited above, wherein the compressed oxidant is supplied by a compressor wheel and flows through the engine in a relatively uniformly shaped annular flowpath, the implementation of means for utilizing the compressed oxidant as a diluent or for convective cooling is relatively straightforward and easily accomplished. There are applications, however, such as in an auxiliary power unit (APU) or an emergency power unit (EPU) for an aircraft, where it is desirable to utilize a gas turbine engine in which the compressor wheel is replaced by a source of gaseous compressed oxidant such as a pressure bottle, and the compressed oxidant, which may be compressed air or oxygen, is fed directly to the combustor via a conduit or duct.
In these applications, which are referred to hereinafter as stored energy power units, implementation of means for utilizing the compressed oxidant as a diluent or for convective cooling are made considerably more difficult by virtue of the fact that the flowpath is neither uniform nor inherently annular and typically requires that a relatively abrupt transition be made between a small diameter cylindrically shaped conduit or duct and a relatively larger diameter annular shaped nozzle.
In the past, two basic design approaches have been utilized for stored energy power units. In the first, a so-called "can" combustor is incorporated into the conduit or duct which supplies the compressed oxidant, and a housing, which functions as a transition duct, is incorporated into the flowpath between the can combustor and the nozzle to guide the hot gases from the can combustor to the nozzle. Examples of various embodiments utilizing this approach are disclosed in U.S. Pat. Nos. 4,343,147, 4,343,148, 4,478,045, 5,009,589, and 5,076,061.
This approach has several major drawbacks. First, the nozzle, shroud, and turbine wheel are not accessible for convective cooling by the compressed oxidant but are directly exposed to the hot gases of combustion, thereby resulting in the necessity for operating the engine with a hot gas temperature below the maximum temperature limits of the materials. Secondly, due to the relatively abrupt expansion of the flowpath which occurs as the hot gases enter the housing from the combustor, it is very difficult to obtain a uniform flow of hot gases along the housing and the shroud to provide convective cooling. This non-uniformity in the flow can lead to the creation of hot and cold spots on the housing and shroud which may in turn cause cracking or burnout to occur. Thirdly, the can combustor protrudes outward from the housing making packaging difficult and requiring excessive volume.
In the second approach, as illustrated by U.S. Pat. Nos. 4,916,893, and 5,060,469, an annular stored energy combustor, similar to the annular combustors described above for U.S. Pat. No. 4,825,640, is utilized. The annular stored energy combustor typically includes a liner defining an annular combustion chamber having an outlet in fluid communication with the nozzle and a housing surrounding and spaced outward from the liner to form a plenum chamber in fluid communication with an oxidant inlet duct. Utilization of such an annular stored energy combustor results in a stored energy power unit which is more compact than a stored energy power unit which uses a can combustor by virtue of the fact that the combustor is contained entirely within the envelope required for the housing. Utilization of the annular stored energy combustor also allows a portion of the compressed oxidant to be used either as a diluent for reducing the temperature of the hot gases or for convectively cooling the engine components, thereby allowing higher combustion temperatures and hence higher power output than is achievable in stored energy power units utilizing can combustors. The problem of maldistribution of the compressed oxidant, resulting from the abrupt transition between the oxidant inlet duct and the nozzle, leading to hot and cold spots which negatively affect reliability and life of the stored energy power unit remain, however, since means for uniformly distributing oxidant flowing from the oxidant inlet duct into the plenum chamber are not provided.
What is needed, therefore, is an improved combustor which includes means for dealing with the abrupt transition in the flowpath which occurs between the conduit or duct and the nozzle, and for providing a uniformly distributed flow of compressed oxidant for use in convective cooling of components in a gas turbine engine receiving gaseous compressed oxidant via a conduit or duct.